1. Field of the Invention
This invention relates to two-component zoom objectives suitable for use in photographic devices.
2. Description Relative to the Prior Art
Among photographic objectives for still cameras, a wide acceptance has been found by zoom lenses having a zoom ratio on the order cf 1.5 to 2.0.
The designs for this type of optical system generally fall into two categories.
In the first category are so-called short zoom lenses that are comprised of a front negative optical unit and a rear positive optical unit, an example of which is disclosed in Japanese Kokai, laid open patent application, 53-91756. Despite the small size of the lens system, these systems have a serious drawback; namely, a long back focal length. Even though the lens itself may be short, a camera embodying the lens is inevitably bulky because of long back focal length.
Although this can be remedied by use of optical folding elements in the space between the lens and ohe film plane, the additional element increases the cost as well as weight of the optical system.
The second category of zoom lenses evolved in order to counter the problems associated with the first lens category. During the search for an approach to come up with small zoom lenses, lens designers had available to them the lenses that are comprised of two optical units of positive and negative power, counting from the front. These lenses have a small back focal length when compared to their focal length and are called telephoto lenses. This means that such lenses have a focal length such that the ratio of the distance from the front vertex to the image plane to the focal length is less than one. Descriptions of telephoto-zoom lenses exist in prior art; for example, FR. 1,259,007 and U.S. Pat. No. 4,682,860. However, these designs tended to consist of at least six lens elements (if color correction is desired) or tended to produce rather steep curvatures of surfaces of lens elements.